Make Particles follow a Black and White image texture
In the image above on the middle see a particle system with an indication at the bottom that a texture is attached.  On the right side you see the location to change the texture and to specify its influence as density.  You press + New to create the texture reference you need. You can ignore the filler on the left side.  This writing is not a tutorial.  Please supplement this with a tutorial as specified above.  This is to show the primary panels to start your process.  You will replace the abstract texture with your texture.  Then if you prefer/need you can invert your texture in the texture panel or invert it before.  So you can flip the colors from black to white and white to black very easily. You can look at the headers of the panels and icons to navigate to the correct place.